The present invention relates to semiconductor devices and, more particularly, to insulated-gate type bipolar transistors (IGBT) acting as power-switching devices.
An insulated-gate type bipolar transistor (hereinafter referred to as IGBT) is a voltage-driven semiconductor switching device capable of high-speed turn-off at a relatively low applied voltage. IGBT's are used widely in the power-electronics field, e.g., in inverters and the like.
An IGBT-output type inverter device may have an overcurrent flow into the IGBT if there is an inrush current when a motor is activated, and failures such as a load short circuit and arm short circuit may occur. Hence, superior electric characteristics are required for protecting the IGBT against high voltage and large current, including a breakdown withstand capability known as a short-circuit withstand capability.
Inverter devices usually include a protection circuit to detect the occurrence of short-circuit failures and to turn the power supply off. This protection circuit requires 10 to 20 .mu.sec to detect the overcurrent and effect its protective function. The IGBT must be protected against breakdown during this time.
Many recent high-performance IGBT modules adopt an overcurrent-protection system which is disposed independently of the protection circuit in the inverter device. Such an overcurrent-protection system can quickly detect an overcurrent flowing into the IGBT when a short-circuit failure occurs, can limit the current in the IGBT, and can suppress it to within the short-circuit withstand capability of the elements by means of a gate control based on this overcurrent detection signal before the power supply is turned off by the protection circuit.
FIG. 5 shows an IGBT overcurrent-protection circuit according to this protection system. Connected in parallel with main element 1 (IGBT) is a current-detection sub-element 2 (IGBT further to the main element 1). The sub-element 2 is also connected in series with a current-detection resistance 3. A switching element 4 (MOSFET) is connected to the gate-driving circuits for the main element 1 and the sub-element 2 to perform on-off operation according to the voltage generated between the ends of the current-detection resistance 3.
When an overcurrent due to load short-circuit failure or the like flows into the main element 1 and the sub-element 2 and causes the voltage between the ends of the current-detection resistance 3 to exceed the threshold voltage of the switching element 4, the switching element 4 turns on to reduce the gate voltage of both the main element 1 and the current-detection sub-element 2, thus limiting the main current flowing in the main element 1. Thus, the main current can be lowered to within the short-circuit withstand capability of the element 1 by means of suitably setting the resistance of the current-detection resistance 3 and the threshold voltage of the switching element 4.
If an overcurrent-protection circuit having a sub-element 2 for current detection is constructed as an external, independent circuit, it is difficult to maintain the operational characteristics of the main element 1 proportional to those of the sub-element 2 because of the temperature rise of the chip. Moreover, since there are different modes of short-circuit phenomena in an inverter, e.g., arm short circuit, series short circuit, output short circuit and ground fault, and since the collector-to-emitter voltage V.sub.CE applied to the IGBT element to be protected may depend on the mode, the current ratio between the main element 1 and the sub-element 2 may vary. Correspondingly, the limited-current value may vary also, if the collector-to-emitter voltage V.sub.CE varies when the operational characteristics of the main element 1 and the sub-element 2 differ as described above. Therefore, it is difficult to realize a stable overcurrent-protection operation.
To solve the above problem, a configuration has been described by the present applicant in Japanese Patent Document No. 5-256197, wherein some of the IGBT cells formed integratedly on a semiconductor substrate are used as sensing cells to detect the current. The emitter electrodes of the sensing cells are separated from the emitter electrodes of the main cells formed on the same substrate, and are connected to a current-detection resistance in an overcurrent-protection circuit.